+1 yep, it is a pretty simple logical task of switching tubes to isolate it to one or a few tubes. From what you said it is in one of your amps… so you can use the tubes in the good one to trouble shoot the one with the noise. Macs are well made so it is the most likely a tube problem.
I always order a spare set of tubes when I get a tube component. You can use these to narrow any tube issues to an individual tube, then get a replacement. Remember the same tubes on a single side should be matched.